Microstrip antennas usually consist of a number of antenna elements and a microstrip distribution network with a ground plane on one side facing towards the antenna elements and a distribution network on the other side. The distribution network sometimes has two separate branches for the connection of two different polarisations of antenna elements. In these different types of antennas and distribution networks unwanted modes arise among other reasons because slots in the ground plane also radiate backwards. Other types of discontinuities also cause unwanted radiation and thereby also unwanted modes. Already-known attempts to solve these problems have involved the introduction of new materials in the laminate of microstrip distribution networks. The article "A Microstrip Array Fed by a New Type of Multilayer Feeding Network", N. I. Herscovici et al., Microwave Journal, July 1995, pp 124-134, describes a method of suppressing unwanted modes by introducing dielectric plugs in the laminate/substrate. The article "Gain enhancement of a thick microstrip antenna by suppressing surface waves", C. S. Lee et al., IEEE AP symposium, 1994, pp 460-463, describes a method of improving microstrip antennas by introducing parasitic elements within the laminate/substrate. These methods are extremely expensive as standard components cannot be used. Introducing new materials into the laminates requires expensive non-standard processing and is therefore not suitable for mass-production.